A step in the fabrication of semiconductor integrated circuits is to apply a thin film of material onto the surface of a semiconductor substrate. The thin film is then patterned to form openings within the film. A second thin film of material is applied to the patterned thin film such that the second thin film fills the openings of the pattern in the first film. The second thin film is then patterned, and a third thin film is applied to the second thin film and is patterned, and the sequence is repeated until the desired integrated circuit structure is created.
The sequence for building an integrated circuit begins with a transistor structure formed on the semiconductor substrate. Alternating layers of electrically conducting and insulating thin film materials are formed over the transistor structure, and the electrically conducting film layers are interconnected to one another to form electrically conducting pathways throughout the integrated circuit. The conducting film material is commonly aluminum or an alloy of aluminum. Vapor deposition is the preferred method for applying the conducting film to the surface of the substrate.
As technology advances toward faster speeds for integrated circuits, the widths of individual lines of the circuitry decrease in size. Although vapor deposition continues to be widely used for depositing films, new methods such as electroplating are being developed for depositing conductor films within tight spacings in a patterned film layer. Additionally, it becomes necessary to use conductors with reduced resistance such as copper due to speed limitations posed by aluminum and alloys of aluminum.
A viable technique for forming a copper thin film layer on a patterned substrate surface is electroplating. A patterned semiconductor substrate is prepared for electroplating. The patterned film on the semiconductor substrate may be an insulating film such as silicon dioxide. The patterned silicon dioxide contains openings to the underlying conductor film material. The underlying conductor film material may be copper or it may be another conductor material. To prepare for electroplating, a seed layer of material may be formed on the underlying conductor film material using vapor deposition.
Once prepared with the necessary seed layer, if any, the patterned semiconductor substrate is placed face down at the top end of an electroplating cup. A cathode contact is created on the edge of the substrate by coupling the substrate to the negative terminal of a power source. There is an anode at the bottom of the electroplating cup. The anode is coupled to the positive terminal of a power source. The substrate is clamped against an O-ring to form a watertight seal around the substrate perimeter. An inlet pipe is inserted into the cup through the bottom of the cup, so that a nozzle at the end of the pipe is inside the cup and faces the substrate surface. A liquid electroplating solution flows through the inlet pipe and out of the nozzle, spraying a liquid jet of fluid directed perpendicularly toward the substrate surface. The electroplating solution contacts the substrate surface, the power supply is turned on, and a circuit is formed between the anode and cathode through the electroplating solution. The desired material is electroplated onto the surface of the substrate.
One aspect of making electroplating a viable process for semiconductor fabrication is to form a uniformly deposited electroplated film layer. Utilizing the standard technique described above, film thickness uniformities of 6% can be achieved. However, for achieving desired process yields an even better uniformity is needed. Moreover, as the substrate size increases from 200 millimeters in diameter to 300 millimeters and beyond, it will be more difficult to attain electroplated film thickness uniformity using the currently known electroplating techniques. There are several factors causing non-uniformity in electroplating. One of the factors is lack of continuity of the cathode contact. This can be corrected by utilizing a cathodic contact ring at the edge of the substrate to form a continuous cathode contact. Another factor causing non-uniform electroplated film thickness is an accumulation of electrolytic solution on surface points on the substrate due to the perpendicular transport of liquid to the substrate surface.
A way of improving electroplated film thickness uniformity was identified in U.S. Pat. No. 4,304,641 “Rotary Electroplating Cell with Controlled Current Distribution”. There the method was to use a flow-through jet plate having nozzles of increasing size and uniformly spaced radially therethrough or the same sized nozzles with varying radial spacing to provide a differential flow distribution of the plating solution. Additionally, the patent disclosed the technique of rotating the substrate by connecting the cathode to a spindle, which in turn is rotated by a motor. Alternatively, the cathode and anode can be rotated at the same time. Rotating the substrate relative to the anode helps to create a more uniform distribution of electroplating solution over the surface of the substrate by preventing accumulation on contact from the liquid spray.
One problem with using a jet plate is that a jet plate must be specially fabricated with exact hole distribution and dimensions. This can significantly add to the cost of fabricating the electroplating equipment. A problem with rotating the cathode and possibly the anode, of course, is the increased complexity of the equipment. Whenever there are moving parts in equipment, equipment maintenance becomes more complex and chances of mechanical failure are greater. Using a motor in the equipment drives the cost of the equipment up. Higher costs are desirably avoided because of the generally increasing costs of producing integrated circuits while prices of manufactured integrated circuit parts continue to decrease.
It is therefore advantageous to use an electroplating technique that can improve the film thickness uniformity while at the same time avoiding increased cost and complexity to the electroplating equipment.